Zoom lens

ABSTRACT

A zoom lens includes sequentially from an object side a first lens group having a negative refractive power; a diaphragm; and a second lens group having a positive refractive power. Zoom from a wide angle edge to a telephoto edge is performed by displacement of the second lens group along an optical axis, toward the object side. Correction of imaging plane variation accompanying the zoom, is performed by displacement of the first lens group along the optical axis. The second lens group includes sequentially from the object side, a positive first lens having at least on aspheric surface and a positive second lens. Furthermore, a first condition υd 21 &gt;63 and a second condition υd 22 &gt;70 are satisfied, υd 21  being the Abbe number for a d-line in the first lens of the second lens group and υd 22  being the Abbe number for a d-line in the second lens of the second lens group.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 2010-040456, filed on Feb. 25,2010, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a zoom lens ideal for video cameras andin particular, surveillance cameras.

2. Description of the Related Art

Conventionally, surveillance cameras, such as those for closed circuittelevision (CCTV) have been used to monitor unmanned facilities.Surveillance cameras capture images during the day using visible lightand at night using near-infrared light. Therefore, a lens system thatcan be used day or night, i.e., a lens system that can handle bothvisible and near-infrared light is demanded for surveillance cameras.

Typically, in a lens system designed for the visible light range,chromatic aberration occurs in the near-infrared light range and imagescaptured at night using near-infrared light are out of focus. Thus, alens system that can correct chromatic aberration over a wide spectrum(from the visible light range to the near-infrared light range) suchthat the focal points of the spectrum become uniform, is preferable foruse in a surveillance camera. A lens that is capable of magnification,is compact, and has a large focal ratio and high optical performance isyet more preferable.

Conventionally, zoom lenses have been proposed that are capable ofhandling light in the visible range to the near-infrared range and aremountable to a surveillance camera (see, for example, Japanese PatentApplication Laid-Open Publication No. 2005-134887). The zoom lensdisclosed in Japanese Patent Application Laid-Open Publication No.2005-134887 includes sequentially from an object side, a first lensgroup having a negative refractive power, a diaphragm, and a second lensgroup having a positive refractive power. The first lens group includessequentially from the object side, 2 negative meniscus lenses, and acemented lens that includes a biconcave lens and a positive lens.Further, the second lens group includes 2 simple lenses disposedfarthest on the object side.

Conventionally, in addition to being able to handle wavelengths over awide spectrum, ranging from the visible light range to the near-infraredlight range, a high focal ratio enabling sharp images to be capturedeven in dimly lit places is also demanded of lens systems forsurveillance cameras. Further, with rapid advances in increasing thepixels of imaging elements (CCD, CMOS, etc.), lens systems capable ofcapturing even finer details of an object, i.e., megapixel lens systems,have also come to be demanded. Therefore, in particular, a megapixellens system for a surveillance camera is demanded that over the entirezoom range, can favorably correct various types of aberration withrespect to light in the visible range to the near-infrared range andthat further has extremely high optical performance.

However, with conventional arts, including the lens system disclosed inJapanese Patent Application Laid-Open Publication No 2005-134887, it isdifficult to maintain high optical performance while satisfying demandfor a higher focal ratio and a smaller size. In other words, if a higherfocal ratio and a smaller size are achieved, the correction of varioustypes of aberration occurring with respect to light in the visible rangeto the near-infrared range becomes difficult, arising in a problem thatpixels cannot be increased on the megapixel order.

To solve the problems associated with the conventional arts above, anobject of the present invention is to provide a megapixel zoom lens thatover the entire zoom range, favorably corrects various types ofaberration occurring with respect to light in the visible range to thenear-infrared range.

SUMMARY OF THE INVENTION

It is an object of the present invention to at least solve the aboveproblems in the conventional technologies.

A zoom lens according to one aspect of the present invention includessequentially from an object side a first lens group having a negativerefractive power; a diaphragm; and a second lens group having a positiverefractive power. Zoom from a wide angle edge to a telephoto edge isperformed by displacement of the second lens group along an opticalaxis, toward the object side. Correction of imaging plane variationaccompanying the zoom, is performed by displacement of the first lensgroup along the optical axis. The second lens group includessequentially from the object side, a positive first lens having at leaston aspheric surface and a positive second lens. Furthermore, a firstcondition υd₂₁>63 and a second condition υd₂₂>70 are satisfied, υd₂₁being the Abbe number for a d-line in the first lens of the second lensgroup and υd₂₂ being the Abbe number for a d-line in the second lens ofthe second lens group.

The other objects, features, and advantages of the present invention arespecifically set forth in or will become apparent from the followingdetailed description of the invention when read in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a cross-sectional view (along an optical axis) of a zoomlens according to a first example;

FIG. 2 is a diagram of various types of aberration at a wide angle edgeof the zoom lens according to the first example;

FIG. 3 is a diagram of various types of aberration at a telephoto edgeof the zoom lens according to the first example;

FIG. 4 depicts a cross-sectional view (along the optical axis) of thezoom lens according to a second example;

FIG. 5 is a diagram of various types of aberration at the wide angleedge of the zoom lens according to the second example;

FIG. 6 is a diagram of various types of aberration at the telephoto edgeof the zoom lens according to the second example;

FIG. 7 depicts a cross-sectional view (along the optical axis) of thezoom lens according to a third example;

FIG. 8 is a diagram of various types of aberration at the wide angleedge of the zoom lens according to the third example; and

FIG. 9 is a diagram of various types of aberration at the telephoto edgeof the zoom lens according to the third example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the accompanying drawings, exemplary embodiments accordingto the present invention are explained in detail below.

A zoom lens according to an embodiment includes sequentially from anobject side, a first lens group having a negative refractive power, adiaphragm, and a second lens group having a positive refractive power.The zoom lens zooms from a wide angle edge to a telephoto edge by movingthe second lens group along an optical axis, toward the object side; andcorrects imaging plane (image location) variations accompanying zoom, bymoving the first lens group along the optical axis.

An object of the present invention is to provide a zoom lens thatachieves a higher focal ratio and a smaller size and that is furtherable to favorably correct, over the entire zoom range, various types ofaberration occurring with respect to light in the visible range to thenear-infrared range.

The second lens group includes from the object side, a positive firstlens having at least 1 aspheric surface, and a positive second lens. Byforming an aspheric surface on the first lens disposed farthest on theobject side in the second lens group, spherical aberration occurringwith a higher focal ratio can be favorably corrected.

In addition, the following conditional expressions are preferablysatisfied, where υd₂₁ is the Abbe number for the d-line in the firstlens of the second lens group, and υd₂₂ is the Abbe number for thed-line in the second lens of the second lens group.υd ₂₁>63  (1)υd ₂₂>70  (2)

Conditional expressions (1), (2) prescribe conditions to favorablycorrect, over the entire zoom range, chromatic aberration occurring withlight in the visible range to the near-infrared range. Use of alow-dispersion material satisfying conditional expressions (1), (2) toform the first lens and the second lens of the second lens group,enables chromatic aberration occurring with light in the visible rangeto the near-infrared range to be favorably corrected over the entirezoom range. Below the lower limits of conditional expressions (1) and(2), axial chromatic aberration becomes difficult to correct, wherebychromatic aberration occurring with light in the visible range to thenear-infrared range cannot be sufficiently corrected.

Further, the following conditional expression is satisfied, where D₂ isthe displacement of the second lens group during zoom from a wide angleedge to a telephoto edge and Z is the zoom ratio.2.25<D ₂ /Z<2.55  (3)

Conditional expression (3) prescribes an appropriate stroke range forthe second lens group during zoom. By satisfying conditional expression(3), high optical performance can be maintained while achieving areduction in the size of the optical system. Above the upper limit ofconditional expression (3), the displacement of the second lens groupincreases, making it difficult to reduce the size of the optical system.On the other hand, although advantageous in terms of size reduction,below the lower limit of conditional expression (3), the correction ofspherical aberration and coma aberration, in particular at the wideangle edge, becomes difficult, arising in a problem of deterioratedoptical performance.

Furthermore, in the zoom lens according to the embodiment, the secondlens group includes on the image plane side of the second lens andsequentially from the object side, a cemented lens that includes apositive third lens and a negative fourth lens, and a positive fifthlens.

Further, in the zoom lens according to the embodiment, the followingconditional expression is preferably satisfied, where υd₂3 is the Abbenumber for the d-line in the third lens of the second lens group.υd ₂₃>70  (4)

Similar to conditional expressions (1) and (2), conditional expression(4) also prescribes a condition to favorably correct, over the entirezoom range, chromatic aberration occurring with light in the visiblerange to the near-infrared range. Use of a low-dispersion materialsatisfying conditional expression (4) to form the third lens of thesecond lens group, enables chromatic aberration occurring with light inthe visible range to the near-infrared range to be even more favorablycorrected over the entire zoom range. Below the lower limit ofconditional expression (4), the correction of axial chromatic aberrationin the third lens becomes difficult.

Furthermore, in the zoom lens according to the embodiment, the firstlens group includes sequentially from the object side, 3 lensesconstituting 3 groups, including a first lens that is a negativemeniscus lens having a convex surface facing toward the object side, asecond lens that is a negative biconcave lens, and a positive thirdlens. Thus, farthest on the object side of the optical system, anegative meniscus lens having a convex surface facing toward the objectside can be disposed which is advantageous in increasing the field ofview.

In addition, the following conditional expression is preferablysatisfied, where υd₁₃ is the Abbe number for the d-line in the thirdlens of the first lens group.υd ₁₃<20  (5)

Conditional expression (5) prescribes a condition that enables chromaticaberration occurring in the first lens group to be corrected by thefirst lens group. In other words, by satisfying conditional expression(5), the third lens, which is a positive lens, causes aberration of thesame magnitude and in the opposite direction of the axial chromaticaberration and chromatic difference of magnification caused by thenegative lens, whereby the first lens group is able to correct chromaticaberration that occurs. Above the upper limit of conditional expression(5), chromatic aberration of a magnitude necessary for correction cannotoccur at the third lens, whereby chromatic aberration occurring in thefirst lens group increases.

As described, the zoom lens according to the embodiment, by satisfyingthe conditional expressions above, in addition to achieving a higherfocal ratio a reduction in size, is able to correct extremely favorably,over the entire zoom range, various types of aberration occurring withlight in the visible range to the near-infrared range. Consequently, thelens is ideal for video cameras, such as surveillance cameras, equippedwith a megapixel imaging element. By simultaneously satisfying pluralconditional expressions, even better optical performance can beachieved, i.e., surpassing that when only conditional expression issatisfied.

FIG. 1 depicts a cross-sectional view (along the optical axis) of thezoom lens according to a first example. The zoom lens includessequentially from an object (non-depicted) side, a first lens group G₁₁having a negative refractive power, a diaphragm STP, and a second lensgroup G₁₂ having a positive refractive power. Between the second lensgroup G₁₂ and an imaging plane IMG, a cover glass CG of an imagingelement is disposed. The cover glass CG is disposed as needed and may beomitted when not necessary. Further, at the imaging plane IMG, the lightreceiving surface of the imaging element, e.g., CCD and CMOS, isdisposed.

The first lens group G₁₁ includes sequentially from the object side, afirst lens L₁₁₁, a second lens L₁₁₂, and a third lens L₁₁₃. The firstlens L₁₁₁ is a negative meniscus lens having a convex surface facingtoward the object side. The second lens L₁₁₂ is a negative biconcavelens. The third lens L₁₁₃ is a positive lens.

The second lens group G₁₂ includes sequentially from the object side, afirst lens L₁₂₁, a second lens L₁₂₂, a third lens L₁₂₃, a fourth lensL₁₂₄, and a fifth lens L₁₂₅. The first lens L₁₂₁ is a positive lens,both surfaces of which are aspheric. The second lens L₁₂₂ is a positivelens. The third lens L₁₂₃ is a positive lens. The fourth lens L₁₂₄ is anegative lens. The third lens L₁₂₃ and the fourth lens L₁₂₄ arecemented. The fifth lens L₁₂₅ is a positive lens.

The zoom lens zooms from a wide angle edge to a telephoto edge by movingthe second lens group G₁₂ along the optical axis, toward the objectside; and corrects imaging plane (image location) variationsaccompanying zoom, by moving the first lens group G₁₁ along the opticalaxis.

Various values related to the zoom lens according to the first exampleare indicated below.

Focal length of entire zoom lens = 3.12 mm (wide angle edge) to 7.70 mm(telephoto edge) F number = 1.05 (wide angle edge) to 1.69 (telephotoedge) Angle of view(2ω) = 118.7° (wide angle edge) to 44.8° (telephotoedge) (Values related to condition expression (1)) Abbe number ford-line in first lens L₁₂₁ in second lens group G₁₂ (υd₂₁) = 81.56(Values related to condition expression (2)) Abbe number for d-line insecond lens L₁₂₂ in second lens group G₁₂ (υd₂₂) = 81.54 (Values relatedto condition expression (3)) Displacement of second lens group G₁₂during zoom from wide angle edge to telephoto edge (D₂) = 5.676 Zoomratio (Z) = 2.468D₂/Z = 2.3 (Values related to condition expression (4))Abbe number for d-line in third lens L₁₂₃ in second lens group G₁₂(υd₂₃) = 81.54 (Values related to condition expression (5)) Abbe numberfor d-line in third lens L₁₁₃ in first lens group G₁₁ (υd₁₃) = 17.98 r₁= 33.7252 d₁ = 0.90 nd₁ = 1.91082 υd₁ = 35.25 r₂ = 7.7780 d₂ = 4.60 r₃ =−20.9605 d₃ = 0.60 nd₂ = 1.51633 υd₂ = 64.14 r₄ = 16.1963 d₄ = 1.46 r₅ =18.5995 d₅ = 1.90 nd₃ = 1.94594 υd₃ = 17.98 r₆ = 57.0799 d₆ = 15.34(wide angle edge) to 2.22 (telephoto edge) r₇ = ∞ d₇ = 6.97 (wide angle(aperture stop) edge) to 1.29 (telephoto edge) r₈ = 19.0568 d₈ = 2.25nd₄ = 1.49710 υd₄ = 81.56 (aspheric surface) r₉ = −65.8596 d₉ = 0.10(aspheric surface) r₁₀ = 15.3317 d₁₀ = 5.00 nd₅ = 1.49700 υd₅ = 81.54r₁₁ = −14.8365 d₁₁ = 0.10 r₁₂ = 24.5794 d₁₂ = 2.80 nd₆ = 1.49700 υd₆ =81.54 r₁₃ = −19.3165 d₁₃ = 0.60 nd₇ = 1.74077 υd₇ = 27.79 r₁₄ = 9.3540d₁₄ = 0.84 r₁₅ = 19.4818 d₁₅ = 2.65 nd₈ = 1.77250 υd₈ = 49.60 r₁₆ =−17.8770 d₁₆ = 1.00 (wide angle edge) to 6.70 (telephoto edge) r₁₇ = ∞d₁₇ = 3.50 nd₉ = 1.51633 υd₉ = 64.14 r₁₈ = ∞ d₁₈ = 4.39 r₁₉ = ∞ (imagingplane) Constant of cone (κ) and Aspheric coefficients (A, B, C, D)(Eighth plane) κ = 1.76524, A = 3.27501 × 10⁻⁵, B = −2.66681 × 10⁻⁶, C =−7.40845 × 10⁻⁸, D = 4.22194 × 10⁻¹⁰ (Ninth plane) κ = −99.89602, A =2.61402 × 10⁻⁴, B = 6.09741 × 10⁻⁷, C = −1.33155 × 10⁻⁷, D = 1.31030 ×10⁻⁹

FIG. 2 is a diagram of various types of aberration at the wide angleedge of the zoom lens according to the first example; FIG. 3 is adiagram of various types of aberration at the telephoto edge of the zoomlens according to the first example. In the diagrams, d-line indicatesaberration for a wavelength equivalent to 587.56 nm; and ΔS and ΔM in aportion depicting astigmatism, indicate aberration with respect to asagittal image plane and a meridional image plane, respectively.

FIG. 4 depicts a cross-sectional view (along the optical axis) of thezoom lens according to a second example. The zoom lens includessequentially from the object (non-depicted) side, a first lens group G₂₁having a negative refractive power, a diaphragm STP, and a second lensgroup G₂₂ having a positive refractive power. Between the second lensgroup G₂₂ and the imaging plane IMG, a cover glass CG of an imagingelement is disposed. The cover glass CG is disposed as needed and may beomitted when not necessary. Further, at the imaging plane IMG, the lightreceiving surface of the imaging element, e.g., CCD and CMOS, isdisposed.

The first lens group G₂₁ includes sequentially from the object side, afirst lens L₂₁₁, a second lens L₂₁₂, and a third lens L₂₁₃. The firstlens L₂₁₁ is a negative meniscus lens having a convex surface facingtoward the object side. The second lens L₂₁₂ is a negative biconcavelens. The third lens L₂₁₃ is a positive lens.

The second lens group G₂₂ includes sequentially from the object side, afirst lens L₂₂₁, a second lens L₂₂₂, a third lens L₂₂₃, a fourth lensL₂₂₄, and a fifth lens L₂₂₅. The first lens L₂₂₁ is a positive lens,both surfaces of which are aspheric. The second lens L₂₂₂ is positivelens. The third lens L₂₂₃ is a positive lens. The fourth lens L₂₂₄ is anegative lens. The third lens L₂₂₃ and the fourth lens L₂₂₄ arecemented. Furthermore, the fifth lens L₂₂₅ is a positive lens.

The zoom lens zooms from a wide angle edge to a telephoto edge by movingthe second lens group G₂₂ along the optical axis, toward the objectside; and corrects imaging plane (image location) variationsaccompanying zoom, by moving the first lens group G₂₁ along the opticalaxis.

Various values related to the zoom lens according to the second exampleare indicated below.

Focal length of entire zoom lens = 3.12 mm (wide angle edge) to 7.70 mm(telephoto edge) F number = 1.05 (wide angle edge) to 1.73 (telephotoedge) Angle of view(2ω) = 118.8° (wide angle edge) to 44.8° (telephotoedge) (Values related to condition expression (1)) Abbe number ford-line in first lens L₂₂₁ in second lens group G₂₂ (υd₂₁) = 71.68(Values related to condition expression (2)) Abbe number for d-line insecond lens L₂₂₂ in second lens group G₂₂ (υd₂₂) = 81.54 (Values relatedto condition expression (3)) Displacement of second lens group G₂₂during zoom from wide angle edge to telephoto edge (D₂) = 6.17 Zoomratio (Z) = 2.468D₂/Z = 2.5 (Values related to condition expression (4))Abbe number for d-line in third lens L₂₂₃ in second lens group G₂₂(υd₂₃) = 81.54 (Values related to condition expression (5)) Abbe numberfor d-line in third lens L₂₁₃ in first lens group G₂₁ (υd₁₃) = 17.98 r₁= 29.3376 d₁ = 0.90 nd₁ = 1.91082 υd₁ = 35.25 r₂ = 7.1781 d₂ = 4.97 r₃ =−20.3779 d₃ = 0.60 nd₂ = 1.51633 υd₂ = 64.14 r₄ = 15.8531 d₄ = 0.65 r₅ =15.2185 d₅ = 1.90 nd₃ = 1.94594 υd₃ = 17.98 r₆ = 38.7199 d₆ = 14.81(wide angle edge) to 3.33 (telephoto edge) r₇ = ∞ d₇ = 7.37 (wide angle(aperture stop) edge) to 1.20 (telephoto edge) r₈ = 19.4930 d₈ = 2.45nd₄ = 1.54332 υd₄ = 71.68 (aspheric surface) r₉ = −85.9542 d₉ = 0.10(aspheric surface) r₁₀ = 17.1478 d₁₀ = 5.00 nd₅ = 1.49700 υd₅ = 81.54r₁₁ = −14.6575 d₁₁ = 0.10 r₁₂ = 20.1365 d₁₂ = 3.05 nd₆ = 1.49700 υd₆ =81.54 r₁₃ = −18.3168 d₁₃ = 0.60 nd₇ = 1.74077 υd₇ = 27.79 r₁₄ = 9.8074d₁₄ = 0.95 r₁₅ = 21.9850 d₁₅ = 2.60 nd₈ = 1.77250 υd₈ = 49.60 r₁₆ =−19.0507 d₁₆ = 1.00 (wide angle edge) to 7.16 (telephoto edge) r₁₇ = ∞d₁₇ = 3.50 nd₉ = 1.51633 υd₉ = 64.14 r₁₈ = ∞ d₁₈ = 4.40 r₁₉ = ∞ (imagingplane) Constant of cone (κ) and Aspheric coefficients (A, B, C, D)(Eighth plane) κ = 1.41938, A = 7.21374 × 10⁻⁶, B = −1.30927 × 10⁻⁶, C =−7.55140 × 10⁻⁸, D = 4.02316 × 10⁻¹⁰ (Ninth plane) κ = 26.45353, A =2.35266 × 10⁻⁴, B = 1.12744 × 10⁻⁶, C = −1.23680 × 10⁻⁷, D = 1.07977 ×10⁻⁹

FIG. 5 is a diagram of various types of aberration at the wide angleedge of the zoom lens according to the second example; FIG. 6 is adiagram of various types of aberration at the telephoto edge of the zoomlens according to the second example. In the diagrams, d-line indicatesaberration for a wavelength equivalent to 587.56 nm; and ΔS and ΔM in aportion depicting astigmatism, indicate aberration with respect to asagittal image plane and a meridional image plane, respectively.

FIG. 7 depicts a cross-sectional view (along the optical axis) of thezoom lens according to a third example. The zoom lens includessequentially from the object (non-depicted) side, a first lens group G₃₁having a negative refractive power, a diaphragm STP, and a second lensgroup G₃₂ having a positive refractive power. Between the second lensgroup G₃₂ and the imaging plane IMG, a cover glass CG of an imagingelement is disposed. The cover glass CG is disposed as needed and may beomitted when not necessary. Further, at the imaging plane IMG, the lightreceiving surface of the imaging element, e.g., CCD and CMOS, isdisposed.

The first lens group G₃₁ includes sequentially from the object side, afirst lens L₃₁₁, a second lens L₃₁₂, and a third lens L₃₁₃. The firstlens L₃₁₁ is a negative meniscus lens having a convex surface facingtoward the object side. The second lens L₃₁₂ is a negative biconcavelens. The third lens L₃₁₃ is a positive lens.

The second lens group G₃₂ includes sequentially from the object side, afirst lens L₃₂₁, a second lens L₃₂₂, a third lens L₃₂₃, a fourth lensL₃₂₄, and a fifth lens L₃₂₅. The first lens L₃₂₁ is a positive lens,both surfaces of which are aspheric. The second lens L₃₂₂ is a positivelens. The third lens L₃₂₃ is a positive lens. The fourth lens L₃₂₄ is anegative lens. The third lens L₃₂₃ and the fourth lens L₃₂₄ arecemented. The fifth lens L₃₂₅ is a positive lens.

The zoom lens zooms from a wide angle edge to a telephoto edge by movingthe second lens group G₃₂ along the optical axis, toward the objectside; and corrects imaging plane (image location) variationsaccompanying zoom, by moving the first lens group G₃₁ along the opticalaxis.

Various values related to the zoom lens according to the third exampleare indicated below.

Focal length of entire zoom lens = 3.12 mm (wide angle edge) to 7.70 mm(telephoto edge) F number = 1.05 (wide angle edge) to 1.69 (telephotoedge) Angle of view (2ω) = 118.8° (wide angle edge) to 44.8° (telephotoedge) (Values related to condition expression (1)) Abbe number ford-line in first lens L₃₂₁ in second lens group G₃₂ (υd₂₁) = 64.14(Values related to condition expression (2)) Abbe number for d-line insecond lens L₃₂₂ in second lens group G₃₂ (υd₂₂) = 81.54 (Values relatedto condition expression (3)) Displacement of second lens group G₃₂during zoom from wide angle edge to telephoto edge (D₂) = 6.06 Zoomratio (Z) = 2.468D₂/Z = 2.45 (Values related to condition expression(4)) Abbe number for d-line in third lens L₃₂₃ in second lens group G₃₂(υd₂₃) = 81.54 (Values related to condition expression (5)) Abbe numberfor d-line in third lens L₃₁₃ in first lens group G₃₁ (υd₁₃) = 17.98 r₁= 30.1359 d₁ = 0.90 nd₁ = 1.91082 υd₁ = 35.25 r₂ = 7.2324 d₂ = 4.67 r₃ =−19.8983 d₃ = 0.60 nd₂ = 1.51633 υd₂ = 64.14 r₄ = 17.0584 d₄ = 0.81 r₅ =16.2597 d₅ = 1.90 nd₃ = 1.94594 υd₃ = 17.98 r₆ = 43.2522 d₆ = 15.18(wide angle edge) to 3.29 (telephoto edge) r₇ = ∞ d₇ = 7.26 (wide angle(aperture stop) edge) to 1.20 (telephoto edge) r₈ = 19.0665 d₈ = 2.40nd₄ = 1.51633 υd₄ = 64.14 (aspheric surface) r₉ = −84.5058 d₉ = 0.10 r₁₀= 16.1865 (aspheric surface) d₁₀ = 5.00 nd₅ = 1.49700 υd₅ = 81.54 r₁₁ =−15.2681 d₁₁ = 0.10 r₁₂ = 19.8426 d₁₂ = 3.05 nd₆ = 1.49700 υd₆ = 81.54r₁₃ = −18.7247 d₁₃ = 0.60 nd₇ = 1.74077 υd₇ = 27.79 r₁₄ = 9.4880 d₁₄ =0.93 r₁₅ = 19.6410 d₁₅ = 2.60 nd₈ = 1.77250 υd₈ = 49.60 r₁₆ = −19.6410d₁₆ = 1.00 (wide angle edge) to 7.06 (telephoto edge) r₁₇ = ∞ d₁₇ = 3.50nd₉ = 1.51633 υd₉ = 64.14 r₁₈ = ∞ d₁₈ = 4.40 r₁₉ = ∞ (imaging plane)Constant of cone (κ) and Aspheric coefficients (A, B, C, D) (Eighthplane) κ = 1.39594, A = 1.01793 × 10⁻⁵, B = −1.98641 × 10⁻⁶, C =−5.82188 × 10⁻⁸, D = 2.44235 × 10⁻¹⁰ (Ninth plane) κ = −32.43286, A =2.33201 × 10⁻⁴, B = 5.39768 × 10⁻⁷, C = −1.08431 × 10⁻⁷, D = 9.49686 ×10⁻¹⁰

FIG. 8 is a diagram of various types of aberration at the wide angleedge of the zoom lens according to the third example; FIG. 9 is adiagram of various types of aberration at the telephoto edge of the zoomlens according to the third example. In the diagrams, d-line indicatesaberration for a wavelength equivalent to 587.56 nm; and ΔS and ΔM in aportion depicting astigmatism, indicate aberration with respect to asagittal image plane and a meridional image plane, respectively.

Among the values for each of the examples above, r₂, . . . indicateradii of curvature for each lens, aperture stop surface, etc.; d₁, d₂, .. . indicate the thickness of the lenses, aperture stop, etc. or thedistance between surfaces thereof; nd₁, nd₂, . . . indicates therefraction index of each lens with respect to the d-line (λ=587.56 nm);and υd₁, υd₂, . . . indicates the Abbe number with respect to the d-line(λ=587.56 nm) of each lens.

Each of the aspheric surfaces above can be expressed by equation [1],where Z is the distance along a direction of the optical axis from theapex of the lens surface, y is the height in a direction normal to theoptical axis, and the travel direction of light is positive.

$\begin{matrix}{Z = {\frac{y^{2}}{{R\left( {1 + \sqrt{1 - {\left( {1 + K} \right){y/R^{2}}}}} \right)}^{2}} + {Ay}^{4} + {By}^{6} + {Cy}^{8} + {Dy}^{10}}} & \lbrack 1\rbrack\end{matrix}$

Where, R is paraxial radii of curvature; K is constant of the cone; andA, B, C, D are the fourth, sixth, eighth, and tenth asphericcoefficients, respectively.

As described above, the zoom lens according to each of the examplesabove satisfies each of the conditional expressions, whereby the zoomlens achieves a high focal ratio and a smaller size while being able tofavorably correct, over the entire zoom range, various types ofaberration occurring with light in the visible range to thenear-infrared range. Furthermore, the zoom lens according to eachexample employs a lens having an appropriately shape aspheric surface,whereby favorable optical performance can be maintained with fewerlenses.

Although the invention has been described with respect to a specificembodiment for a complete and clear disclosure, the appended claims arenot to be thus limited but are to be construed as embodying allmodifications and alternative constructions that may occur to oneskilled in the art which fairly fall within the basic teaching hereinset forth.

1. A zoom lens comprising sequentially from an object side: a first lens group having a negative refractive power; a diaphragm; and a second lens group having a positive refractive power, wherein zoom from a wide angle edge to a telephoto edge is performed by displacement of the second lens group along an optical axis, toward the object side, correction of imaging plane variation accompanying the zoom, is performed by displacement of the first lens group along the optical axis, the second lens group includes sequentially from the object side, a positive first lens having at least one aspheric surface and a positive second lens, and a first condition υd₂₁>63 and a second condition υd₂₂>70 are satisfied, υd₂₁ being the Abbe number for a d-line in the first lens of the second lens group and υd₂₂ being the Abbe number for a d-line in the second lens of the second lens group.
 2. The zoom lens according to claim 1, wherein a third condition 2.25<D₂/Z<2.55 is satisfied, D₂ being displacement of the second lens group during the zoom from the wide angle edge to the telephoto edge and Z being a zoom ratio.
 3. The zoom lens according to claim 1, wherein the second lens group includes on an image plane side of the second lens and sequentially from the object side, a cemented lens that includes a positive third lens and a negative fourth lens, and a positive fifth lens, and a fourth condition υd₂₃>70 is satisfied, υd₂₃ being the Abbe number for a d-line in the third lens of the second lens group.
 4. The zoom lens according to claim 1, wherein the first lens group includes sequentially from the object side, 3 lenses constituting 3 groups, including: a first lens that is a negative meniscus lens having a convex surface facing toward the object side, a second lens that is a negative biconcave lens, and a positive third lens, and a fifth condition υd₁₃<20 is satisfied, υd₁₃ being the Abbe number for d-line in the third lens of the first lens group. 